Fix ghost button on payment step for subscriptions

What does this MR do and why?

Fix subscription and addons purchase flow - remove the "ghost" button on payment section

Related #344038 (closed)

Screenshots or screen recordings

Mind that zuora iframe might looks different for production but the main button in the left bottom corner is independent from the iframe. The screenshot for After section is from my local environment

Before	After

How to set up and validate locally

• Subscription purchase flow

1. Navigate to a GitLab.com group that does not a seat subscription and you are the group Owner of
2. Navigate to Settings > Billing and upgrade to a paid plan
3. Fill out the first two sections to get to the payment method section.

Fast-forward link -> http://localhost:3000/-/subscriptions/new?plan_id=2c92c0f97700a4f70177060963d923c8

• Addon purchase flow
  1. Run CustomersDot locally
  2. On your local GitLab instance enable a feature toggle
     • Run rails c
     • Run Feature.enable(:new_route_storage_purchase) to enable the feature flag for Storage flow
     • Run Feature.enable(:new_route_ci_minutes_purchase) to enable the feature flag for CI Minutes flow
  3. Navigate to http://localhost:3000/-/subscriptions/buy_storage?selected_group=<id_of_an_existing_group>
  4. Navigate to http://localhost:3000/-/subscriptions/buy_minutes?selected_group=<id_of_an_existing_group>

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.